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MAX485 ground isolation

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ehsantech

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Hello,
In my circuit I used two different adapter to isolate my ARM from my remote driver(with an optocoupler)(Two different ground wires are available). Now I want to add Max485 chip to my circuit to get encoder data. My main problem is with ground wire. Regarding this circuit, I connect adapter2 +V,GND to my encoder input voltage, and connect adapter +V,GND to feed the max485. Is it necessary to have a shared GND between my max485 and encoder? in this mode I should connect both GND to each other which disables my isolation that I created with my optocoupler.

Max.png

what is your suggestion?
 

wwfeldman

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redraw the circuit

draw a line down the center of your page

put the motor and encoder on the right,
put the controller on the left
put the optocoupler in the middle - the controller side on the controller side etc
put the adapters where they belong - one on each side, if I followed your information

the max485 is a RS-485. RS-422 transceiver
based on where from and where to it transmits, chooses a side and use an optocoupler, or a transformer, or ??? as needed
power it from the adapter on the same side
 

KlausST

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Hi,

Is it necessary to have a shared GND between my max485 and encoder?
Usually yes, because you need to ensure that the RS485 signals are within specified range referenced to it's GND.

It should be mentioned in every RS485 IC datasheet as well as RS485 application notes.
This should always be your first places to look for related informations.

Klaus
 

kripacharya

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Ground plane grids on opposite sides of a optocoupler should not be connected. You have to find a way to avoid this. If you cannot, then delete the optoisolator from your circuit because it serves no real purpose.
 

ehsantech

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Hello back,
Here is my circuit with high resolution.
Capture-rs.JPG
Based on inductive noise, short-circuit, ... I have to use optocoupler to isolate my micro-controller from high-voltage. However for Max485 it seems I should connect both GND to each other.
 

asdf44

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There are many isolated RS-485 solutions which could solve a lot of problems including some new integrated power ICs:

MAXM22511

And search "isolated RS-485"
 

kripacharya

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Hello back,
Here is my circuit with high resolution.
View attachment 153654
Based on inductive noise, short-circuit, ... I have to use optocoupler to isolate my micro-controller from high-voltage. However for Max485 it seems I should connect both GND to each other.
then you are a fool. making stupid mistakes. No one can help you.
 

wwfeldman

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you aren't paying attention
the dotted line separating the two sides must be continuous and
there can be no physical connection from one side to the other

no wires may cross the boundary

only optical couplers, transformers and the like

you cannot tie the grounds together
 

ehsantech

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Hello,
Thank you for all useful help.
Thank you for your suggestion, I was thinking about the isolated one as you recommended. But it is more expensive for my application.
kripacharya
Can you let me know my mistakes ?
wwfeldman
Thank you for your explanation. But I didn't understand well what you said.
the dotted line separating the two sides must be continuous and there can be no physical connection from one side to the other
It means I should connect both GND and GND1 to each other for serial connection. But
you cannot tie the grounds together
what does it mean?

- - - Updated - - -

In addition,if you were me, what would you do?

1)Did you use opto-coupler? for industrial environment in this structure?
2)How would it be your wiring?

- - - Updated - - -

As I want to learn more and improve myself. I be grateful if you could help me step by step.
By using this structure, this circuit will work fine with a good isolation from both power supply or I missed something?
1.JPG
In this structure,
1)if I connect GND3 to GND4 what will happen?
2)if I use only one adapter(use only a 24V adapter and Use for all inputs( opto-coupler,motor,MinMax dc-dc) what will happen? as MinMax has a high isolation, still do I have good isolation in my circuit?
3)if I connect GND3 to GND what will happen?
 
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wwfeldman

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Inkedattachment_LI.jpg

note dotted line separates the controller side from the motor side

replace the max 485 part with maxm22511 as suggested by asdf44
it costs more, as you said, but it may make the difference between working and not working
it will need power from each adapter - power for the motor side, and power for the controller side

where does the output of the encoder/485 go?

connecting the grounds together will introduce the electronic noise
from the motor into the controller and can disrupt its operation

gnd3 and gnd2 from your diagram in post 9 can be tied together
they are on the same side of the isolation line

the gnd on the motor side cannot be tied to the gnd on the controller side
because it will un-isolate the two parts of your circuit
 

ehsantech

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Dear wwfeldman, I appreciate your kindness and taking your time to answer my question. Yes, I have found another isolated max485 and a dc-dc converter to feed it and I hope it facilitates my problem( I thank you and asdf44)
where does the output of the encoder/485 go?
In my diagram, for simplification, I just showed data lines only by two wires.(it is AB,YZ input and output)
I changed the circuit to this one.In minmax datasheet it mentioned it isolates input and output completely(1.5Kv) Would it be a good solution?
4.JPG
 

asdf44

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Yes, isolation can solve a lot of problems.

To be clear the maxm22511 includes an isolated DC-DC.

If you already have a power source on each side choose a cheaper isolated 485 IC such as ISO308x

Heed wwfeldman's suggestions when it comes to drawing these diagrams - draw the entire isolation barrier very clearly. I still see wires crossing it....
 

ehsantech

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Thank you again for everything.
How about the earth wire, where should I consider it?
 

KlausST

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Hi,

Which Earth wire?
Did you show one?

BTW: I recommend to use different colors for supply lines and signal lines. And please show the DCDC converter as isolating device with the dotted lines.


Klaus
 

ehsantech

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Thank you KlausSt for your help everywhere in this forum.
6.JPG
I want to know where is the appropriate place for earth wire to put. Because the dc-dc converter make the voltage floating. So by using the SMPS and a dc-dc converter where is the best place for earth.

Thanks
 

KlausST

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Hi,

If you just want to prevent it from floating, then add a 1M anywhere you like.
Check voltage rating of the resistor.

Klaus
 
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